One of the most common forms of email is provided by Email Service Providers (ESPs) such as Yahoo! Mail, Microsoft Hotmail, Google GMail, and other web-based email services. In large scale ESPs, the problem of unwanted bulk email messages (UBE), or also known as spam, is acute. Providers use a number of techniques in an effort to shield users from receiving UBE.
Additionally, ESPs may themselves be unwilling conduits for UBE senders. Spam senders constantly derive new and different methods of both usurping inbound message safeguards, and efforts by ESPs to thwart spammers using their systems as conduits for SPAM. UBE senders may, for example, change the characteristics of their emails to usurp spam email filters as such filters learn the characteristics of specific messages. In addition, spammer may try to send large volumes of messages within a short period of time so that the maximum number of messages bypass spam filters before being detected.